The invention relates generally to optical transport systems and more particularly to an optical fiber transmission system with an adaptive phase prechirp transmitter.
Optical networks are presently the physical medium of choice and maximizing their performance is a constant need. The universal access to communications created by the introduction of Internet has driven the demand for more high-bandwidth networks capable of handling large volumes of data at high speed and low latency.
A Composite Signal Degradation Factor
For transmitting data over an optical network, the information is encoded into a series of pulses, or bits. As the signal propagates over the physical fiber, degradations of the signal tend to accumulate with distance and can make a pulse unrecognizable above the ambient noise level in the optical channel. Such degradations in signal quality can cause bits to be misinterpreted at the receiving end, thereby increasing the bit error rate (BER).
Signal degradation is due to factors like dispersion, chirped lasers, chirped external modulators, just to enumerate a few. Dispersion, or pulse broadening, is the main contributor to signal degradation. By broadening the pulses, this phenomenon causes inter symbol interference (ISI) where widened pulses encroach on time slots of neighbouring pulses.
The phenomenon of group velocity dispersion (GVD), e.g. chromatic dispersion (CD) and polarization mode dispersion (PMD), as well as fiber non-linearities, e.g. the self-phase modulation (SPM), are significant obstacles to achieving quality communication standards over longer repeaterless transmission distances.
Dispersion causes pulse broadening due to the fact that different optical wavelengths travel at different speeds within a fiber depending on the fiber parameters. Chromatic dispersion (CD) is a signal degradation effect which increases linearly with the length of the optical path. Chromatic dispersion (CD) can be also viewed as variations in the propagation constant of the fiber in respect to the frequency.
With the advent of optical amplification, modern transport networks generally operate in the 1,540 nm window of silica fiber, while the low-dispersion window is at 1,310 nm. However, most of optical fiber transmission lines presently laid are 1,310 nm, low-dispersion, single mode fibers (SMF). The effect of dispersion can be reduced if special fibers with minimal dispersion characteristics shifted from the 1,310 nm window to 1,540 nm window are employed. Such fibers are referred to as dispersion shifted fibers (DSF). However, even for DSF, the accumulated dispersion across the wavelength band at 1,000 km reach is far beyond a 10 Gbps receiver dispersion tolerance.
PMD causes pulse broadening because the two orthogonal polarization modes of the light travel at different speeds along a fiber. This is mainly due to the ellipticity of the fiber core. In addition, the distribution of signal energy over the different states of polarization (SOP) changes with time, due to changes in ambient conditions, e.g. temperature changes, and thus the PMD penalty varies with time as well. For example, the differential time delay between the two orthogonal SOPs on a link is usually between 0.5 and 2.0 ps/√km, and may vary over the bandwidth of a source.
The optical signal is also degraded by the Kerr effect which is a non-linear effect of the optical transmission medium, representing the increase of the index of refraction of the fiber with the intensity (I) of the optical signal. The changes of this index modulate the phase of the optical signal passing through the fiber and thus, re-distributes the signal frequency spectrum. Self-phase modulation (SPM) is characteristic for systems where the optical signal modulates itself, and the resulting changes in frequency distribution are translated into amplitude modulation due to fiber dispersion.
The interplay between CD and non-linearities such as SPM, can lead to increased distortion as a function of transmission distance. The combined GVD-SPM effect for waveform degradation may be expressed as:DB2PavL2=constant,  EQ1where D is the dispersion (ps/nm/km);B is the bit transmission rate (bps);Pav is the average optical power (mW) in the transmission line; andL is the transmission distance.
For example, if the transmission rate is increased from 10 Gbps to 40 Gbps, Pav has to be increased 4-times, resulting in a reduction of the transmission distance of 1/64 L for the same allowable dispersion (D).
It is known that some isotropic materials when they are under stress, e.g. mechanical forces, thermal forces, electrical fields, become anisotropic and may change the index of refraction in certain directions within the fiber material. Dispersion-compensating elements (DCE) having a negative induced dispersion coefficient, are typically introduced in the path of light to create negative dispersion to counteract the positive dispersion experienced in signal propagation through optical fibers.
A Dispersion Compensation technique usually implies use of successive lengths of fiber with positive and negative alternating dispersions, or dispersion slopes [S=dD/dλ) ], for controlling the dispersion phenomenon over a long span. The single mode (SM) optical fiber causes positive dispersion (D) at a rate of +17 pico seconds per kilometer per wavelength of light, or D=17 ps/(nm-km). If a SM fiber is alternated along a span with dispersion compensating fiber (DCF), having a negative dispersion value larger than the dispersion of the SM fiber, a small net link dispersion may be obtained for the entire connection. For example, fiber spans with high negative dispersion D=−68 ps/(nm-km) used in coils of ¼ L, are sufficient to compensate conventional SM fiber and to cause “back-to-zero”, or dispersion equalization.
Large negative dispersion may be also introduced using chirped fiber Bragg gratings, Mach-Zender interferometers (MZI), Gires-Tournois interferometers (GTI), etc.
To maintain quality communications with higher bit rate signals, the optical signal-to-noise ratio (OSNR) must increase to overcome the effect of the ISI. Such increase in OSNR is often obtained by increasing the transmit power and is accounted as the “power penalty”. The group velocity dispersion (GVD) limits the permissible link length (L) for a given power penalty. Moreover, when increasing the optical power, the self-phase modulation (SPM) effect increases as well.
Other non-linear phenomena in fiber are due to photons interaction with atoms which may result in atoms being excited to higher energy levels. When excited, atoms are not stable and tend to return to lower energy levels by releasing photons. These released photons propagate at a velocity that depends on their energy [W=f(λ)] and therefore, their wavelength is different from that of the originating photon.
The Chirp Parameter
As an almost monochromatic light-pulse travels trough a fiber, its amplitude variations cause phase changes (ΔΦ) and spectral broadening. The phase changes (ΔΦ) depend on wavelength and the modulation technique used. For a LiNbO3 laser the phase variation is given, according to the equation:
                    ΔΦ        =                  C          ·                      π            2                    ·                                    V              0                                      V              π                                                  EQ2      where is the modulation voltage, is the voltage for full modulation [(0,1)] or Mach Zehnder, and “C” is the chirp parameter [−1<C <+1]. These phase changes are equivalent to frequency modulation, or “chirping”. Significant spectral broadening, or pulse form degradation, is observed when ΔΦ≧2.
The emergency of the ultra long haul (ULH) optical transport networks where optical signals travel long distances without regeneration, places new demands on the dispersion-limited distances. The configuration of the paths and the evolution of the network may cause existing DCEs to be inadequate, and not easy to allocate along the optical path. In addition, the existing adaptive DCEs have a slow response to a rapidly changing network.
A significant factor in the cost associated with optical transport systems is the number of regenerators (OEO) required along a given communications link within the network. It is important to increase the distance between such regenerators and to reduce their number along a given communication link. The distance between regenerators is at least in part governed by the degradation of the signal along the optical path, and such signal degradation is reflected by the Bit Error Rate (BER) value, which is the ratio between the number of erroneous bits counted at the receiver (Rx) site over the total number of received bits, over a given period of time.
In addition, known dispersion compensation techniques operate “broadband” over a set of wavelengths. Broadband dispersion compensators, in general, do not equalize the dispersion at every wavelength, therefore impacting transmission performance, particularly at the edges of the wavelength transmission window. This is mainly due to the fact that these broadband compensators do not compensate precisely for dispersion slope (SO).
There is a need for a mechanism for dynamically compensating/eliminating signal degradations in the received signal for performing channel optimization in order to achieve higher bit rates and longer transmission spans.